Current regulators for higher currents, whose output current is a function of an input quantity, are hard to control in respect to their dynamic stability because of the great steepness and the resulting high amplifications of open collector stages. High-frequency oscillations over the entire range of the operating current under all conditions of use are hard to prevent, in particular in connection with rapidly operating regulators, which are intended for universal use and as a consequence may be connected with the operational voltage, the control device and the consumer via cable harnesses of different length.
Because the transition frequency of transistors is reduced with lowering current density, it would be required to make the upper limit frequency of the positive feedback loop for the range located in the vicinity of the output current "zero" lower than for the range of higher currents However, variable limit frequencies require increased circuit outlay.
From the textbook by Winfried Oppelt, "Kleines Handbuch technischer Regelvorgaenge" [Brief Manual of Technical Regulation Processes], published by Verlag Chemie GmbH, Weinheim [Fed. Rep. of Germany], Vol. 5, 1972, as well as from French patent document FR-A-2 336 841, non-linear regulators are already known, whose describing function or transfer function has a dead zone in the range around the zero point.
From German patent document DE-A-1 513 127 a circuit arrangement for avoiding error functions of an electrical main circuit is furthermore known, which prevents the switching on of a main circuit prior to the required supply voltage having been reached. For this purpose a control and amplifier unit is placed upstream of the main circuit which only switches on the main circuit after a required supply voltage has been reached. From the article by Claude Boisard entitled "Un CI regulateur de vitesse pour moteur a courant continu a aimant permanent" [A Printed Circuit Speed Control for a Constant Current Motor with a Permanent Magnet] in the magazine "Electronique Industrielle et Microelectronique (EMI)", No. 162 of Oct. 15, 1972, pp. 57 to 60, a speed control for an electric motor with a permanent magnet is furthermore known, which contains in the motor excitation circuit an npn-end stage transistor whose emitter is connected to a negative supply voltage. In this case an operational amplifier is placed upstream of the npn-end stage transistor for switching the npn-end stage transistor on and off as a function of a control quantity and a reference voltage.
Furthermore, from FIG. 4 of German Letters Patent DE-PS 21 47 179 and corresponding U.S. Pat. No. 4,028,564, a current regulator in accordance with the species of the main claim is known, identified there as a controllable current source, which is designed for low output currents.